(1) Field to which invention relates
The present invention relates to an energy beam engraving method for the production of pit-shaped depressions of different dimensions in the surface of a workpiece, in the case of which depressions with smaller and, respectively, larger dimensions are produced by shorter and, respectively, longer lasting action of an energy beam, more particularly a charge carrier beam and preferably of an electron beam with a pre-established and substantially constant power, which is focussed in a plane at the surface to be engraved.
Furthermore the invention relates to an apparatus for carrying out such a method.
(2) The prior art
The German Patent Specification (Auslegeschrift) No. 1,123,561, the U.S. Pat. No. 3,246,079, the East German Patent Specification No. 55,965 and the German Patent Specification (Offenlegungsschrift) No. 2,111,628 have already described methods for the production of gravure or intaglio forms by means of an electron beam, with which pit-shaped depressions (in what follows referred to as "pits" for short) of the most various different shapes can be produced. In this respect the following parameters of the energy-beam can be controlled: beam intensity, the size and shape of the hot spot, the position of the hot spot, intensity distribution, movement of the hot spot and length of action. In the case of the production of smaller and larger pits of a predetermined type however generally only one of these parameters is varied as for example the intensity or the duration of action of the charge carrier beam. The variation in the beam sharpness or focus, which is described in the German Patent Specification No. 1,123,561 is practically equivalent to a change in cross-section.
Furthermore the German Patent Specification (Offenlegungsschrift) No. 2,207,090 refers to the production of round and rhombic pits by means of a charge carrier beam with an elliptical cross-section, in the case of which the axial ratio can be controlled by means of a stigmator. The cross-sectional area of the engraving charge carrier beam is generally substantially smaller than that of the pit to be engraved. During engraving the operation is started at one edge or at the one corner of the pit with a practically punctuate focussing of the charge carrier beam and the matter is then deflected over the area of the pit and distorted in an elliptical or line-like manner up to a maximum size and then reduced in size again.
The engraving of the pits becomes substantially simpler as regards complexity of apparatus and the control of the apparatus if the energy-beam does not need to be deflected during engraving a predetermined pit and if in the case of all pit sizes it is possible to operate with the same beam power. If however, as is suggested for example in the East German Patent Specification 55,965, the beam power is held constant and the varying dimensions of the pits are only produced by a suitable control of the duration of action, it is necessary in practice to make compromises as regards the beam parameters and such compromises involve substantial disadvantages. More especially in the case of small pits there is a substantial formation of flash at the edge of each pit and the bottoms of the pits become irregular. Furthermore there is a tendency of the pit diameter to become too small in the case of light shades.